Magnetic Recording and Playback 1057that retain the low-frequency output capability of older
tapes, but utilize new equalization curves optimized for
the new tape thickness.
28.3.2.2 Fringing
Spacing loss is evident not only at high frequencies, but
it also shows up at very long wavelengths. Magnetic
information that is recorded off to the sides or fringes of
the area normally scanned by the reproduce head core
will begin to be sensed if the wavelength becomes
longer than the separation distance. At studio operating
speeds of 15 in/s and 30 in/s (38 cm/s and 76 cm/s), for
which low-frequency wavelengths reach ½ inch and
1 inch (12.7 mm and 25 mm), this fringing leakage
becomes very evident. For example, for the case of an
oversized record track mentioned in Section 28.2.4.1,
the signal level may rise by the ratio of the track width
to the core width. A typical 24 track format on 2 inch
(51 mm) tape would encounter a 1.3 dB rise at frequen-
cies below 500 Hz for record and reproduce cores of
43 mils and 37 mils (1.1 mm and 0.9 mm).
A similar case arises when alignment tapes made
with a single full-width record head are utilized for
level and response checks. The sideways fringing will
produce significant level and response errors. The actual
amounts of error depend on both the track format and
the playback head design. Some alignment tape manu-
facturers roll off the low frequencies in an attempt to
offset the rise in a nominal head, but the amount of this
fringing compensation is not absolutely correct for all
head designs.
One additional pitfall to be avoided is the fringing
differences between center tracks and edge tracks. Since
the edge cores run very near the physical edge of the
tape, these cores sense only one-half the amount of
fringing flux sensed by the inner cores. During a
frequency check from a full-width alignment tape, the
two edge tracks should therefore be slightly lower in
output at the low frequencies than the remainder of the
tracks.
28.3.2.3 Contour Effect
At very low frequencies, the wavelength of the recorded
signal may become as long as the magnetic core of the
playback head. These long wavelengths enter the core at
the gap and at the sides and rear of the core. The
resulting flux in the core will consist of the desired flux
from the gap plus additions and/or subtractions of the
fringing flux leaking into the core at the sides and back.
The voltage output of the head, which is dependent on
the net flux coupled into the windings, will undulate at
low frequencies as the wavelengths create varying
levels of constructive and destructive interference due
to the fringing flux.
The response curve in Fig. 28-19 illustrates the
nature of the undulations or head bumps for a typical
mastering recorder at 15 in/s (38 cm/s) and 30 in/s
(76 cm/s) using a reproduce head that has a 0.5 inch
(12 mm) core face. Two well-defined head bumps are
usually evident for such mastering heads. The bumps
shift up an octave in frequency for each doubling of
tape speed, creating an even more severe problem at
30 in/s (76 cm/s).Heads with either very small cores or only a small
window in the head shielding at the gap area can
produce numerous ripples in the low-frequency
response. Such heads should be avoided unless the tape
speed is slow enough to avoid serious problems within
the normal band of audio frequencies.
The exact shape of the head bumps is determined by
the size and shape of the reproduce core, surrounding
shielding material, and angle of wrap of the tape. Since
the user cannot adjust these parameters during the
normal alignment procedure, the bumps can only be
modified by adding an outboard equalizer, which
cancels the bumps with an inverse response curve.
Recent improvements in the control of head bumps
has reduced the magnitude of the bumps in present-day
mastering recorders to less than 1 dB peak-to-peak at
15 in/s (38 cm/s) and 1.5 dB peak-to-peak at 30 in/sFigure 28-19. Contour effect. Courtesy Sony Corporation
of America.+20
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Frequency HzAmplitudedB15 in/s 30 in/s